A conventional facsimile machines or the like uses an image sensor as an optical reader of original images. The image sensor generally includes a plurality of photodiodes aligned along the width of the original, each of the photodiodes having a semiconductor layer of an amorphous material or the like interposed between a pair of electrodes.
The conventional image sensor is illustrated in FIG. 10 in a cross-sectional view, and has a laminated configuration including an insulating substrate 1 of, for example, glass or ceramics, a lower electrode 2 of chromium (Cr), a semiconductor photoelectric conversion layer 3 of amorphous silicon hydride or the like, and a transparent electrode 4 of indium tin oxide. A light emitting device (not shown) emits light, which is reflected by the original document, etc. (not shown), passes through the rod lens array (not shown) such as SelFoc lens manufactured by Japan Sheet Glass Co. Ltd, and is finally focussed on the photoelectric conversion layer 3 through the transparent electrode 4.
The thus-configured image sensor usually uses as image information the electric charge generated by light incident on the photoelectric conversion layer 3, instead of using the photocurrents resulting from the light, since the light illuminating the photoelectric conversion layer 3 is approximately 100 lux in the usual optical mechanism mentioned above. Light of this intensity generates photocurrents of approximately 1 nA, which are too small to detect. The image sensor therefore is usually operated using a charge-reading method, in which the electric charge generated in the photoelectric conversion layer 3 is stored in a first storage element such as a capacitor. The stored charge is then transferred, using switching elements such as thin-film transistors, to a second storage element, such as a capacitor, then read out as an electric signal.
This charge-reading method will be now described in detail with reference to FIG. 11, which shows an equivalent circuit diagram of a portion of the conventional image sensor corresponding to one picture element. Each photoreceptor element, which corresponds to one picture element, may be regarded as equivalent to a photodiode PD and a series-connected capacitor CPD. In operation, the photodiode PD is first reverse-biased at +5 V, and then illuminated to cause a photocurrent flow in the photoelectric conversion layer, which is stored as an electric charge in the capacitor CPD. In response to the energization of an associated field effect transistor T, the stored charge is transferred to a floating capacitor CL in an associated wiring conductor at the bottom of the figure. An external amplifier l then amplifies the transferred charge and outputs an electric signal. After the electric signal is output, a resetting analog switch S resets the circuit.
The above image sensor suffers from the following problems. The capacitance of capacitor CPD of the image sensor is relatively small and generally insufficient. This limited capacitance deteriorates the linearity of the proportional relationship between the exposure (=illumination intensity.times.time) and the output voltage of the image sensor. Insufficient capacitance coincident with poor photo-/dark current ratio cause poor gradation in the electric signal.
In Japanese Patent Laid Open No. SHO. 61-82570 (1986), an image sensor is disclosed which has the capacitance of its capacitor increased by extending the photoelectric conversion layer in the slow scan direction so as to enlarge the portion of the photoelectric conversion layer interposed between the lower electrode and the upper electrode. The photoreceptive area of the image sensor is limited by shielding.
This image sensor, which is illustrated in FIG. 12 in cross-sectional view and in FIG. 13 in perspective view, includes a plurality of photoreceptor elements aligned on a substrate 1 in the fast scan direction. The respective electrodes 5 of the photoreceptor elements extend in the slow scan direction from the tops of the photoreceptor elements to a shift register to introduce the electric charge generated in the photoreceptor elements to the shift register.
Each of the photoreceptor elements has a laminated configuration, including an insulating substrate 1, a lower electrode 2 as a common electrode and formed of chromium (Cr), a photoelectric conversion layer 3 of amorphous silicon hydride or the like extending in the slow scan direction such that a relatively wide portion of the photoelectric conversion layer 3 overlaps with the lower electrode 2, a transparent electrode 4 of indium tin oxide formed over the photoelectric conversion layer 3, and an extracting electrode 5 of chromium partially covering the transparent electrode 4 so as to define the photoreceptive area.
The above configuration creates two capacitors CPD1 and CPD2. Capacitor CPD1 is formed by the lower electrode 2 and the transparent electrode 4, and CPD2 is formed by the lower electrode 2 and the extracting electrode 5, as shown in FIG. 14 of an equivalent circuit diagram of the photoreceptor element. The photoreceptor element having capacitors with increased capacitance can store more electric charge, improving the linearity of the proportional relationship between the exposure and the output voltage of the image sensor.
However, this conventional image sensor shown in FIGS. 12, 13 and 14 has a relatively poor photo-/dark current ratio. By enlarging the photoelectric conversion layer 3 to increase the capacitance of the capacitors, the dark currents in the photoelectric conversion layer 3 are increased, resulting in a lower photo-/dark current ratio.